1. Field of the Invention
The present invention relates in general to devices for measuring a distance and more particularly, to distance measuring devices of a laser beam using type which, for measuring the distance between the device and an object, transmits a laser beam toward the object and detects a returning laser beam which has been reflected by the object. In practical use, the period required for the laser beam to make the round trip is detected. The distance to the object is equal to the one-half of the time elapsed multiplied by the velocity of the laser beam.
2. Description of the Prior Art
In the above-mentioned conventional distance measuring devices, there is a type which generally comprises a case having a window opening covered with a flat glass, a laser beam emitter installed in the case, a returning laser beam detector installed in the case, a four-sided polygon mirror installed in the case and rotated about a first axis for reflecting a going laser beam from the laser beam emitter toward a remote object and reflecting a returning laser beam from the object toward the returning laser beam detector, and a nodding mechanism installed in the case for pivoting the four-sided polygon mirror about a second axis which is perpendicular to the first axis. Both the going laser beam from the polygon mirror and the returning laser beam from the object pass through the flat glass window of the case.
However, due to the nature of the flat glass, it tends to occur that a part of the going laser beam from the polygon mirror is largely reflected by the flat glass toward an optical track along which the returning laser beam travels to the returning laser beam detector. In fact, the reflection of the laser beam by the flat glass becomes severe when, due to the work of the nodding mechanism, the incidence angle of the laser beam to the flat glass increases. Of course, under such condition, a precise measurement of the distance is not achieved due to interference of the going laser beam with the returning laser beam.